code: def test_expression_function_nostring():
      # expr() can only work with str otherwise
      # should raise a SchemaValidationError
      with pytest.raises(ValidationError):
          expr(2 * 2)
  
      with pytest.raises(ValidationError):
          expr(["foo", "bah"])
  
file_path: altair/tests/expr/test_expr.py
imports: import operator
  import pytest
  from altair import expr
  from altair import datum
  from altair import ExprRef
  from jsonschema.exceptions import ValidationError
label_desc: Use image node_id 13 for calling a global function with example usage: test_expression_function_nostring() without return types